Has anybody tried implementing Google analytics on authoring environment itself. Yes, sounds crazy. However as aem OOTB doesn't provide any information related to concurrent users and active logged in users, pages authored most etc.., thought why not implement GA on authoring environment.
I'm agree with the colleagues which suggest you an other way to do this.
It depends on the version that you are using, but e.g. in 6.3 you can use the out of the box User Report (etc/reports/userreport) in order to get part of this information.
Anyway, if you want to get data by using GTM I think that you just only need to identify the call that need to be tracked (e.g. j_security_check for login, /editor.html for editing, and so on) and ask to the GTM team to track this. From your side you just only need to extend the cq.authoring clientlib in order to load the GTM scripts.
Mind that this could cause some performance issues in author, and my suggestion is to evaluate if it's something really necessary since (as already discussed) you can get this information also from report and logs.
For data related to logged in users , you could simply fetch it from <host>:<port>/system/console/tokenmgr
For Page audits, Its stored under "/var/audit/com.day.cq.wcm.core.page/content/we-retail". You can query for events such as "pageModified" Or "pageDeleted" Or "pageAdded". The primary type of the node will be "